Size triangular and rectangular valley gutters for roof drainage per AS/NZS 3500.3:2021. Calculates flow depth, wave motion allowance, and rainwater head dimensions.
Enter catchment details and roof geometry to size a triangular valley gutter formed by two equal-slope roof planes.
Note: This tab assumes equal roof slopes on both sides of the valley (90° building corner angle). For unequal slopes or box gutters, use the Rectangular Box Valley tab.
Enter catchment details and gutter geometry to size a rectangular box valley gutter.
Note: Box valley gutters are typically used on commercial buildings where higher flow capacity is needed. The gutter slope is set independently of the roof slope.
Catchment area and rainfall intensity determine the total flow the valley must carry.
Flow depth is solved iteratively using Manning's equation for open channel flow, accounting for channel shape and slope.
Wave motion allowance (50% of flow depth) and freeboard ensure the valley handles real-world conditions including wind and debris.
Our hydraulic engineers design roof drainage, stormwater, and overflow systems for commercial and residential projects.
This valley gutter calculator helps hydraulic engineers, roofers, and building designers size valley gutters where two sloping roof surfaces meet at an internal angle. Select your project location for the design rainfall intensity, enter the catchment area and valley gutter dimensions, and the calculator will determine the required flow depth, wave motion allowance, freeboard, and total gutter depth. The tool supports both triangular valley gutters (formed by the intersection of two roof planes) and rectangular box valley gutters.
Calculations use Manning's equation for open channel flow with a roughness coefficient of n = 0.014 for sheet metal, as specified in AS/NZS 3500.3:2021. The flow depth is solved iteratively based on the channel shape, slope, and design flow rate. A wave motion allowance of 50% of the calculated flow depth is added to account for turbulence at the valley, plus a minimum freeboard. The calculator also sizes rainwater heads at the outlet based on the downpipe diameter.
This tool is suitable for standard valley gutter applications on residential and commercial roofs. For valleys with long run lengths, multiple contributing roof areas, or complex junctions where valleys intersect, consult a qualified hydraulic engineer. Final designs should account for leaf guard provisions, maintenance access, and compliance with local council and certifier requirements for roof drainage.